Field of the Invention
Embodiments of the present invention generally relate to a field of transparent display technology, more particularly, relate to an organic electroluminescent display substrate with an improved light transmittance, a manufacturing method thereof, and a display device comprising the organic electroluminescent display substrate.
Description of the Related Art
Transparent display is a new display technology, which enables an observer to observe a background behind a display screen through the screen, thereby extending application fields of the display to display devices, such as a television, a cellphone, a laptop, a display window, a refrigerator door, a vehicle display, a billboard and so on.
An organic electroluminescent device, such as an organic light-emitting diode (OLED), including an active matrix organic light-emitting diode (AMOLED), has characteristics, such as self-illumination, high luminance, high resolution, wide visual angle, fast response speed, low power consumption, capability of being applied to flexible display and so on, and it is a display device which facilitates realizing a transparent display. In the organic electroluminescent device, in order to facilitate realizing a transparent display, each pixel unit in the organic electroluminescent device comprises a light-emitting region and a non-light-emitting region, wherein an electroluminescent structure is formed in the light-emitting region, and non-light-emitting regions of parts of pixel units can be used to realize the transparent display.
With regard to the OLED display device, there are mainly two manners to increase a transparency of the display device so as to realize the transparent display. One manner is to change a structure of a pixel circuit by reducing an area of non-transparent layers or structures (for example, active layer and metal wires) as much as possible, so that an area of a transparent window is increased to increase the transparency of the display panel. The other manner is to replace the metal wires and a cathode of the device to be manufactured with transparent conductive material so as to increase the transparency of the panel. A transparent cathode is very important during preparing the transparent OLED, because it is not only related with performances of the device, but also affects the transparency particularly. In a top-emitting device, the transparent cathode is typically made of thin metal or metallic oxide. The metallic oxide has a high transmittance, however, a sputtering process which is used to form the metallic oxide cathode is apt to damage the OLED device. The thin metal cathode is prepared through an evaporation process, which is simpler and is easy for mass production, so that many designs employ thin metal or metal alloy (for example, Mg and Ag alloy) to manufacture the cathode, and the transmittance of the panel may be increased, for example, by adjusting a doping ratio of Mg and Ag.
In the electroluminescent device in the prior art, however, there are various organic and inorganic protective layers or insulation layers. Though these layers are transparent in the non-light-emitting region, they still cause light loss and transmittance reduction in the non-light-emitting region, leading to a poor transparent display effect of the organic electroluminescent device. The transmittance of the device may be increased by removing a part of or all the organic and inorganic protective layers in the non-light-emitting region, however, a large gradient will be generated at an interface between the light-emitting region and the non-light-emitting region due to a difference in film thickness, thus the thin layer cathode may be broken at the interface, causing the device cannot be activated. Moreover, the transmittance of the light-emitting region will be affected if increasing the thickness of the cathode.